Cyclohexanone oxime is an important intermediate in the industrial production of caprolactam. Cyclohexanone oxime is typically produced by reacting cyclohexanone with hydroxylamine phosphate, and its production status will directly affect the production cost and yield of caprolactam.
FIG. 1 is a schematic diagram showing a conventional recycling system for hydroxylamine formation and oximation. As shown in FIG. 1, the conventional recycling system comprises a hydroxylamine formation tower (10), an oximation tower (30), an extraction tower (50), a stripping tower (70) and a nitric acid absorption tower (90). In the system, inorganic process solution containing nitrate ions and hydrogen gas are delivered, respectively, via lines 101 and 103 to the hydroxylamine formation tower (10), where hydroxylamine phosphate is synthesized. Unreacted hydrogen gas and other gases formed are discharged via a line 105. The inorganic process solution containing hydroxylamine phosphate is delivered via a line 111 to the oximation tower (30) by feeding from the top, and an organic phase solution containing cyclohexanone is delivered via lines 113 and 115 to the bottom of the oximation tower (30). The two solutions feeding from the opposite directions contacted with each other to carry out oximation reaction. The organic phase containing the produced cyclohexanone oxime is discharged to the top of the oximation tower (30) via a line 117, while the residual phosphate-containing inorganic process solution is discharged from the bottom of the oximation tower (30) via a line 119. The discharged phosphate-containing inorganic process solution is delivered via the line 119 to the extraction tower (50), where the residual cyclohexanone oxime is removed; then, the phosphate-containing inorganic process solution is delivered via a line 125 to the stripping tower (70), where the phosphate-containing inorganic process solution is stripped to further remove the residual organic impurities. Finally, the stripped phosphate-containing inorganic process solution is delivered via a line 127 to the nitric acid absorption tower (90), where the inorganic process solution is supplemented with nitrate ions, and then the phosphate-containing inorganic process solution is recycled to the hydroxylamine formation tower (10) for use in hydroxylamine phosphate synthesis in the next cycle.
However, the conventional recycling system has a problem, namely, it is difficult to enhance the yield per unit time, if the design of the oximation tower remains unchanged. For example, when the inorganic process solution containing hydroxylamine phosphate and the organic solution containing cyclohexanone are fed into the oximation tower in an increased amount and/or an increased concentration of hydroxylamine and cyclohexanone contained therein, when the yield of cyclohexanone oxime per unit time is increased, the efficiency of oximation may decrease if the oximation tower is overloaded. As a result, the yield cannot be effectively increased. To overcome the above problem, one can increase the number or the volume of the oximation tower or change the mode of operation. However, the former is limited by the production cost and compliance with regulations (installment of a new oximation tower is expensive; the height of the original equipment cannot be unlimitedly increased); and the latter may cause problems in the way of feeding. For example, as disclosed Chinese patent publication No. 1424306, an oximation process with high efficiency, wherein the solution of cyclohexanone was fed into the oximation tower in a reduced amount from the bottom of the oximation tower, while it was fed in an increased amount from the meddle part of the oximation tower, thereby lowering the organic compounds (including cyclohexanone and cyclohexanone oxime) in the phosphate-containing inorganic process solution discharged from the bottom of the oximation tower. However, in case that the concentration or the flow rate of the inorganic process solution containing hydroxylamine phosphate fed into the oximation tower and/or the concentration of the hydroxylamine phosphate therein are greatly increased, changes in the position where the cyclohexanone stream is fed may result in incomplete oximation, which would lead to increasing in the total carbonyl content of the phosphate-containing inorganic process solution discharged from the bottom of oximation tower, and in turn, inactivate the catalyst in the hydroxylamine formation tower.
Accordingly, it is necessary to develop a process for producing cyclohexanone oxime, which not only provides enhanced efficiency of oximation and increase yield of cyclohexanone oxime, but also lowers the organic content in the inorganic process solution discharged from the oximation tower, even if the amount of feeds for the oximation tower and/or the concentration of hydroxylamine phosphate in the feed are greatly increased.